1. Field of the Invention
The present invention relates to a photodetector used to detect very weak optical signals. More specifically, the invention relates to an ultra-high sensitivity, high-performance, high-speed and low-power consumption photodetector that operates in the ultraviolet, visible and infrared regions, which are important in the fields of scientific instruments, telecommunications and general consumer products.
2. Description of the Prior Art
The various designs for photodetectors may be categorized into photodiodes in which photocarriers at a semiconductor photosurface are simply transformed into photo-current, avalanche photodiodes in which photocarriers are accelerated and multiplied, photo-FETs in which photocarriers are accumulated underneath the gate regions and amplified, and photo-multiplier tubes in which photoelectrons are multiplied. The silicon photodiode is most suitable and less expensive in the spectra range of 0.4 to 1.0 μm for use in scientific measurements. In contrast, photo-multiplier tubes have been used for ultraviolet region, and photo-multiplier tubes or InGaAs photodiodes have been used for infrared region. However, photo-multiplier tubes are bulky and cannot be made into arrays. They also need high voltage power supplies. Photodiodes need sense amplifiers which induce additional electronic noise, and their S/N (signal-to-noise) ratio is low.
For high-sensitivity physical measurement of such wavelengths, devices are frequently cooled down to prevent degradation of sensitivity induced by a thermal noise, which causes additional complexity in the apparatus and increased production costs. Accumulation of photocarriers by long-term exposure is another method of improving the S/N ratio that is commonly applied in silicon CCDs (charge-coupled devices). However, this cannot improve the S/N ratio in short exposure times.
In contrast, the avalanche photodiode and PIN diode are commonly used in telecommunication systems. The avalanche photodiode has a multiplication effect and is so sensitive that single photon detection is possible at cryogenic temperatures. However, 1 G bit/s has been the maximum bit rate, limited by the recovery time from the avalanche multiplication of the photogenerated electrons. For current communication systems which require a detection rate of more than 10 G bit/s, an optical signal is first amplified with an optical fiber amplifier and detected with an even faster PIN photodetector in a complex system. Such complexity is inevitable in the system with the PIN diode because the quantum efficiency of the PIN diode is physically limited to 1 (approximately 1 A/W): one photon generates only one pair of electron and hole.
For such reasons, a high-speed and highly sensitive photodetector with a response speed greater than 10 GHz and with a multiplication effect is necessary for optical communication systems. Single photon detection with a count rate greater than 100 MHz is another requirement to realize a quantum-cryptographic communication system.
Phototransistors are also produced for generaluse equipment as position sensors and optical power monitors with response speeds of 1 microsecond. In conventional phototransistors, photogenerated carriers are accumulated in the base region of the bipolar transistor and effectively change the base current of the transistor. Similarly in the MOS FET, photogenerated carriers are accumulated underneath the gate region and modulate the majority current, and this phenomenon is effectively employed for the photodetectors with the multiplication effect. They are called ‘photo-MOS FET’ and widely used for optically isolated relays. That is, amplification is realized by accumulating the photogenerated charge and reading out as a majority carrier current of the FET. Although some instability in the current is induced by the fluctuation of the accumulated carrier location, the problems of reading amplifier noise and parasitic capacitor have been avoided. Sensitivity and speed generally have a reciprocal relationship. That is, the response speed deteriorates as the sensitivity increases. In the extreme case, persistent photoconductivity due to deep levels occurs at cryogenic temperatures in the GaAs/AlGaAs FET. Its sensitivity goes up close to infinity, although it is not easy to control its performance. Improving both sensitivity and response speed is a difficult task in general. However, micro-fabrication techniques can effectively reduce the device's capacitance and resistance and improve its performance.
Now that single photon detectors and single photon emitters must operate at a relatively high speed of 100 MHz, the phototransistor is promising as the structural foundation for expanding the range of operating wavelengths, and also achieving dramatic improvements in integration, sensitivity, response speeds and reduced power consumption. The following studies are related to the above demands for expanded operating wavelengths, increased sensitivity, higher speeds and lower power consumption.    (a) Non-patent reference document 1: A. J. Shields, M. P. Ritchie, R. A. Hogg, M. L. Leadbeater, C. E. Norman, and M. Pepper, “Detection of single photons using a field-effect transistor gated by a layer of quantum dots,” Appl. Phys. Lett. Vol. 76, No. 25, (June 2000) 3673–3675.). Here, single photons are detected from the compound semiconductor modulation doped FET with quantum dots formed between the channel and gate electrode.    (b) Non-patent reference document 2: Masashi Shima, Yoshiki Sakuma, Yuji Awano, and Naoki Yokoyama, “Random telegraph signals of tetrahedral-shaped recess field-effect transistor memory cell with a hole-trapping floating quantum dot gate,” Appl. Phys. Lett. Vol. 77, No. 3, (2000) 441–443.) Here, a single charge memory device is disclosed using a quantum dot at the bottom of a tetrahedral recess as a charge accumulation layer and an adjacent quantum well at the side wall as a charge detection FET, respectively.    (c) JP-A HEI 9-260711. Here, a constricted electron channel is formed in a modulation doped structure either by gate electrodes or regrowth of semi-insulating layers, and the accumulation of holes in the constricted channel realizes high-sensitivity photodetectors.    (d) Non-patent reference document 3: Akira Fujiwara, Kenji Yamazaki, and Yasuo Takahashi, “Detection of single charges and their generation-recombination dynamics in Si nanowires at room temperature,” Appl. Physics Lett. Vol. 76, No. 25, (June 2000) 3673.” This paper suggests that quantum wire is sensitive as a single-charge detector.
In the device disclosed in non-patent reference document 1, detection of induced charge is inefficient because the conducting channel of the FET is two-dimensional.
In non-patent reference document 2, although it is advantageous that charge accumulation and charge read out regions are constructed three-dimensionally, the use of oblique side walls on the higher-order substrate restricts the selection of constituent materials, limits circuit design, and causes high power dissipation in the read-out FET due to poor electron mobility along the channel.
In JP-A HEI 9-260711, it is difficult to reduce the size of the constricted electron channel in the modulation doped structure less than 0.1 micron either by gate electrodes or regrowth of semi-insulating layers, although concept of the structure is desirable. Large parasitic capacitance becomes an obstacle for the high speed operation of photodetectors as well. The regrowth process induces interface defects especially in the aluminum containing materials, such as AlGaAs, which cause instability in device operation.
The device disclosed in non-patent reference document 3 is immature as a practical device, although it carries new possibilities for silicon based photodetectors.
The present invention has as its object to realize a new, high-sensitivity, wide spectral range and low power consumption photodetector with low production costs, by employing one-time selective growth.